1. Field of the Invention
This invention relates to a perpendicular magnetic recording medium, and preferably relates to a perpendicular magnetic recording medium mounted in various magnetic recording devices including the external recording devices of computers.
2. Description of the Related Art
Two magnetic recording methods, which are an in-plane magnetic recording method and a perpendicular magnetic recording method, are used in hard disk devices, magneto-optical recording devices (MO), magnetic tape devices, and other magnetic recording devices. For both of these methods, there are two important issues to improve recording densities.
The first issue is that of reducing the magnetostatic coupling forces between magnetic crystal grains so that signals can be written in minute regions. For example, in in-plane magnetic recording media and in early perpendicular magnetic recording media, material systems based on Co with Cr added were widely used. Employing these materials, magnetic phase separation is promoted by heating the substrate during thin film formation, and so it is possible to form Cr-rich regions and Cr-poor regions within the thin film. Cr is nonmagnetic, and by adjusting the composition a structure can be realized in which Cr-poor ferromagnetic crystal grains are surrounded by Cr-rich nonmagnetic regions. A. Murayama and M. Miyamura, J. Appl. Phys., Vol. 76, pp. 5361-70 (1994) and S. Iwasaki and K. Ouchi, IEEE Trans. Magn., Vol. 14, pp. 849-851 (1978) disclose that through use of this method, magnetic characteristics are improved and read/write characteristics are enhanced.
There are also examples in which, without using phase separation by substrate heating, a nonmagnetic body which is completely insoluble is intermixed with a ferromagnetic material, to realize a granular structure in which ferromagnetic crystal grains are forcibly surrounded by a nonmagnetic material. For example, cases have been studied in which an oxide, of which SiO2 is representative, is added to a Co-base material system, or C is added to a material system based on Fe; various such methods are disclosed in T. Oikawa et al, IEEE Trans. Magn., Vol. 38, pp. 1976-1978 (2002) and J. S. Chen et al, J. Appl. Phys., Vol. 103, pp. 07F517 1-3 (2008). The above-described structures are standard structures in recent high-density magnetic recording media, and in particular in perpendicular magnetic recording media.
The second issue is that of accommodating writing of signals to minute regions by reducing the sizes of magnetic crystal grains. In order to make magnetic crystal grains smaller in size, reducing the crystal grain size of the layer immediately therebelow (the underlayer) is effective, and various underlayers have been studied (see for example T. Oikawa et al, IEEE Trans. Magn., Vol. 38, pp. 1976-1978 (2002) and W. K. Shen et al, J. Appl. Phys., Vol. 97, pp. 10H301 1-3 (2005)). In addition to magnetic layers, it has also been disclosed that magnetic characteristics are improved by adding various materials to the underlayer to induce a granular structure (I. Takekuma et al, J. Appl. Phys., Vol. 99, pp. 08E713 1-3 (2006)).
In I. Takekuma et al, J. Appl. Phys., Vol. 99, pp. 08E713 1-3 (2006) presented as an example above, a method is disclosed in which, by adding SiO2 to Ru to realize an underlayer with a granular structure, a perpendicular magnetic recording medium with improved characteristics and in which reduced magnetostatic interaction between and smaller sizes of magnetic crystal grains are realized. And as indicated in Y. Inaba et al, IEEE Trans. Magn., Vol. 41, pp. 3136-3138 (2005), R. H. Victora et al, IEEE Trans. Magn., Vol. 41, pp. 537-542 (2005), and D. Suess et al, Appl. Phys. Lett., Vol. 87, pp. 012504 1-3 (2005), in recent years methods have been used in which numerous layers with different characteristics are stacked to improve the magnetic recording medium characteristics. Further, as described in R. Mukai et al, IEEE Trans. Magn., Vol. 41, pp. 3169-3171 (2005), in order to obtain desired characteristics in the magnetic layer, often an underlayer with a multilayer stacked structure is adopted. If the magnetic layer or the underlayer is configured using two layers, the lower-portion layer behaves as an underlayer with respect to the upper-portion layer. Hence in order to raise the recording density of a magnetic recording medium, a quantitative understanding of the effects of granular material characteristics on the microstructure and consequently on read/write characteristics, and simplification of material selection, are sought.